Process for the production of carboxylic acids

ABSTRACT

Aliphatic carboxylic acids of formulae (Ib) R 1 —COOH (Ia) and R 2 —COOH wherein R 1  is tertiary C 4-20 -alkyl and R 2  together are —(CH 2 ) n — with n=3 to 10, are produced by oxidizing an aliphatic or alicyclic ketone of the formula (II):  
                 
 
wherein R 1  and R 2  are as defined above, with molecular oxygen in the presence of a soluble manganese (II) compound. The process has a high selectivity and is carried out under very mild conditions. It is especially useful for the production of dicarboxylic acids, such as, adipic acid, from cyclic ketones.

The invention relates to a process for the production of carboxylicacids by the oxidation of aliphatic or alicyclic ketones.

Aliphatic carboxylic acids and their derivatives are compounds ofparticular industrial interest. In particular, dicarboxylic acids suchas adipic acid, 1,8-octanedioic acid (suberic acid) or1,12-dodecanedioic acid are valuable starting materials for thesynthesis of polyamides and other polymeric materials.

Several syntheses of dicarboxylic acids starting from cyclic ketones areknown in the art, some of them being performed on a commercial scale.The most commonly used oxidant is nitric acid, although some otheroxidants have been investigated, too. The nitric acid oxidation hasseveral drawbacks, resulting mainly from the corrosivity of nitric acidand the byproducts formed, in particular nitrous oxide which has beenfound to cause environmental problems.

The technical problem to be solved by the present invention was toprovide a selective and high-yield process for the production ofaliphatic carboxylic or dicarboxylic acids from aliphatic orcycloaliphatic ketones using a non-corrosive and environmentallyfriendly oxidant.

According to the present invention, this problem has been solved by theprocess of claim 1.

It has been found that aliphatic carboxylic acids of the generalformulaeR¹—COOH   (Ia)andR²—COOH   (Ib)

-   wherein R¹ is tertiary C₄₋₂₀-alkyl-   and R² is selected from hydrogen and C₁₋₆-alkyl,-   or R¹ and R² together are —(CH₂)_(n)— with n=3 to 10,    can be prepared by oxidizing ketones of the general formula-   wherein R¹ and R² are as defined above,    with molecular oxygen in the presence of a soluble manganese(II)    compound.

Here and hereinbelow, C₁₋₆-alkyl is to be understood as any linear orbranched primary, secondary or tertiary alkyl group having 1 to 6 carbonatoms, in particular groups such as methyl, ethyl, propyl, isopropyl,butyl, isobutyl, sec-butyl, tert-butyl, pentyl, neopentyl, hexyl and soon.

Tertiary C₄₋₂₀-alkyl is to be understood as any tertiary alkyl grouphaving 4 to 20 carbon atoms, in particular groups such as tert-butyl(1,1-dimethylethyl) or tert-pentyl (1,1-dimethylpropyl).

The process according to the invention is applicable to the oxidation ofopen-chain (aliphatic) ketones as well as alicyclic ketones, the formeryielding a pair of monocarboxylic acids and the latter yielding adicarboxylic acid.

In a preferred embodiment, R¹ and R² together are —(CH₂)_(n)— and n isan integer selected from 3, 4, 5, 6 and 10, corresponding to thealicyclic ketones cyclopentanone, cyclohexanone, cycloheptanone,cyclooctanone and cyclododecanone as starting materials and thedicarboxylic acids glutaric acid (1,5-pentanedioic acid), adipic acid(1,6-hexanedioic acid), pimelic acid (1,7-heptanedioic acid), subericacid (1,8-octanedioic acid) and 1,12-dodecanedioic acid as products.

In another preferred embodiment, R¹ is tert-butyl and R² is hydrogen,the starting material being methyl tert-butyl ketone and the products(Ia) and (Ib) pivalic acid and formic acid, respectively.

Preferably, the soluble manganese(II) compound is manganese(II) nitrate.Other manganese(II) salts such as manganese(II) acetate may also beused.

More preferably, manganese(II) nitrate is used in an amount of 0.1 of 4mol %, based on the amount of ketone (II).

The conversion rate and selectivity of the oxidation may be furtherimproved by adding at least one compound selected from the groupconsisting of nitric acid, soluble cobalt(II) compounds and solublecopper(II) compounds in an amount of 0.1 to 5 mol %, based on the amountof ketone (II).

Preferably, the oxidation reaction is carried out in a solventcomprising at least one C₂₋₅-alkanoic acid. More preferably, acetic acidis used as solvent.

In a preferred embodiment, the molecular oxygen is used either inessentially pure form or in a gaseous mixture containing up to 90 vol %of nitrogen, e. g., ordinary air. Preferably, the total pressure isabout 1 bar (i. e., ambient pressure) to 20 bar, more preferably about 1bar to 5 bar.

The reaction temperature is preferably 0° C. to 100° C., more preferably20° C. to 70° C. As the oxidation is exothermic, external cooling may benecessary to avoid thermal excursions.

Preferably, the oxidation reaction is carried out in a solvent, theweight ratio solvent/ketone being from 4 to 20 (i. e., 4:1 to 20:1).Besides the preferred alkanoic acids, any solvent which is essentiallyinert under the reaction conditions may be employed.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1 Adipic Acid

A solution containing 25 mmol of cyclohexanone, 0.5 mmol ofmanganese(II) nitrate, 0.5 mmol of cobalt(II) nitrate and 1.5 mmol ofnitric acid in 25 ml of acetic acid was stirred for 5 h at 40° C. in anoxygen atmosphere under ambient pressure. Subsequently, the acetic acidwas distilled off. The residue was found to consist mainly of adipicacid with some glutaric acid and a small amount of unreacted startingmaterial. The conversion of cyclohexanone was found to be 97.5%, theselectivity of the oxidation to adipic acid being 93.4%.

EXAMPLES 2-8, COMPARATIVE EXAMPLES 1-3 Adipic Acid

The procedure of Example 1 was repeated with different catalysts(without addition of nitric acid) at 20° C. or 40° C. The conditions andresults are shown in Table 1. TABLE 1 Example No. Catalyst [mmol]Temperature [° C.] Conversion [%] Selectivity [%] 2 Mn(NO₃)₂ / 0.5 2068.7 92.2 Co(NO₃)₂ / 0.5 3 Mn(NO₃)₂ / 0.5 20 34.6 81.7 4 Mn(OAc)₂ / 0.520 9.1 83.1 5 Mn(NO₃)₂ / 0.5 40 90.6 89.1 Co(NO₃)₂ / 0.5 6 Mn(OAc)₂ /0.5 40 19.3 90.1 Co(OAc)₂ / 0.5 7 Mn(NO₃)₂ / 0.5 40 91.6 89.5 Cu(NO₃)₂ /0.5 8 Mn(OAc)₂ / 0.5 40 17.1 91.2 Cu(OAc)₂ / 0.5 C1 Co(NO₃)₂ / 0.5 206.2 80.3 C2 Co(OAc)₂ / 0.5 20 trace — C3 Cu(NO₃)₂ / 0.5 20 trace —

EXAMPLE 9 Adipic Acid

The procedure of Example 1 was repeated using 50 mmol of cyclohexanoneinstead of 25 mmol. The conversion of cyclohexanone was 95.7%, theselectivity 91.8%.

EXAMPLE 10 Adipic Acid

The procedure of Example 1 was repeated using air at 5 bar instead ofoxygen at 1 bar. The conversion of cyclohexanone was 95.7%, theselectivity 92.6%.

EXAMPLES 11-14

The procedure of Example 1 was repeated using several different ketonesas starting materials and a reaction temperature of 60° C. The resultsare compiled in Table 2. TABLE 2 Conversion Selectivity Example No.Ketone [%] Product [%] 11 Cyclopentanone 98.2 Glutaric acid 91.7 12Cycloheptanone 99.1 Pimelic acid 92.6 13 Cyclooctanone 97.3 Suberic acid93.4 14 Cyclododecanone 98.6 1,12- 92.3 Dodecane- dioic acid

EXAMPLE 15 Pivalic Acid

The procedure of Example 1 was repeated using methyl tert-butyl ketoneinstead of cyclohexanone and 70° C. reaction temperature. The conversionwas 89.6%, the selectivity 96.4%. Formic acid was formed as secondproduct (Ib).

1. A process for the production of aliphatic carboxylic acids of thegeneral formulaeR¹—COOH   (Ia) andR²—COOH   (Ib) wherein R¹ is tertiary C₄₋₂₀-alkyl and R² is selectedfrom hydrogen and C₁₋₆-alkyl, or R¹ and R² together are —(CH₂)_(n)— withn=3 to 10, comprising oxidizing a ketone of formula

wherein R¹ and R² are as defined above, with molecular oxygen in thepresence of a soluble manganese(II) compound.
 2. The process of claim 1,wherein R¹ and R² together are —(CH₂)_(n)— with n=3, 4, 5, 6 or
 10. 3.(canceled)
 4. The process of claim 3, wherein the soluble manganese (II)compound is manganese (II) nitrate.
 5. The process of claim 4, whereinthe amount of manganese (II) nitrate is 0.1 to 4 mol % with respect tothe amount of ketone (II).
 6. The process of claim 5, wherein at leastone additive selected from the group consisting of nitric acid, solublecobalt (II) compounds and soluble copper(II) compounds is used in anamount of 0.1 to 5 mol % with respect to the amount of ketone (II). 7.The process of claim 6, wherein the oxidation is carried out in asolvent comprising at least one C₂₋₅-alkanoic acid.
 8. The process ofclaim 7, wherein the molecular oxygen is used in essentially pure formor in a gaseous mixture containing up to 90 vol % of nitrogen and thetotal pressure is about 1 to 20 bar.
 9. The process of claim 8, whereinthe reaction temperature is 0 to 100° C.
 10. The process of claim 9,wherein a solvent is present in an amount corresponding to a weightratio solvent/ketone of 4 to
 20. 11. The process of claim 1, wherein thesoluble manganese (II) compound is manganese (II) nitrate.
 12. Theprocess of claim 11, wherein the amount of manganese (II) nitrate is 0.1to 4 mol % with respect to the amount of ketone (II).
 13. The process ofclaim 12, wherein at least one additive selected from the groupconsisting of nitric acid, soluble cobalt (II) compounds and solublecopper (II) compounds, is used in an amount of 0.1 to 5 mol % withrespect to the amount of ketone (II).
 14. The process of claim 2,wherein the soluble manganese (II) compound is manganese (II) nitrate.15. The process of claim 14, wherein the amount of manganese (II)nitrate is 0.1 to 4 mol % with respect to the amount of ketone (II). 16.The process of claim 15, wherein at least one additive selected from thegroup consisting of nitric acid, soluble cobalt (II) compounds andsoluble copper (II) compounds, is used in an amount of 0.1 to 5 mol %with respect to the amount of ketone (II).
 17. The process of claim 1,wherein the oxidation is carried out in a solvent comprising at leastone C₂₋₅-alkanoic acid.
 18. The process of claim 1, wherein themolecular oxidation is used in essentially pure form or in a gaseousmixture containing up to 90 vol % of nitrogen, and the total pressure isabout 1 bar to 20 bar.
 19. The process of claim 1, wherein the reactiontemperature is 0 to 100° C.
 20. The process of claim 15, wherein thereaction temperature is 20 to 70° C.
 21. The process of claim 1, whereina solvent is present in an amount corresponding to a weight ratiosolvent/ketone of 4 to
 20. 22. The process of claim 9, wherein thereaction temperature is 20 to 70° C.
 23. The process of claim 1, whereinthe reaction temperature is 20 to 70° C.
 24. A process for theproduction of aliphatic carboxylic acids of formulae:R¹—COOH   (1a) andR²—COOH   (1b) wherein R¹ and R² together are —(CH₂)_(n)— and n is 3 to10, consisting of oxidizing a ketone of formula:

wherein R¹ and R² are as defined above, with molecular oxygen, inessentially pure form or in gaseous mixture with nitrogen, in thepresence of a member selected from the group consisting of (a) a solublemanganese (II) salt, a soluble cobalt (II) salt and nitric acid, (b) asoluble manganese (II) salt and nitric acid, and (c) a soluble manganese(II) salt, a soluble copper (II) salt and nitric acid, and optionally inthe presence of at least one solvent.